<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>css 垂直居中</title>
</head>
<body>
<header>
    <h3>css 垂直居中</h3>
</header>
<main>
    <fieldset>
        <legend>hhh</legend>
    </fieldset>
</main>
</body>
</html>
